The term of copyright for a particular work depends on several factors, including whether it has been published, and, if so, the date of first publication. As a general rule, for works created after January 1, 1978, copyright protection lasts for the life of the author plus an additional 70 years. For an anonymous work, a pseudonymous work, or a work made for hire, the copyright endures for a term of 95 years from the year of its first publication or a term of 120 years from the year of its creation, whichever expires first. For works first published prior to 1978, the term will vary depending on several factors. To determine the length of copyright protection for a particular work, consult chapter 3 of the Copyright Act (title 17 of the United States Code). More information on the term of copyright can be found in Circular 15a, Duration of Copyright, and Circular 1, Copyright Basics.

I'm saying I'm seeing a sudden plethora of '50s jazz albums (and other genres TBH) which is a good thing. I don't really care why.

To me it's always seemed copyright laws are way too complicated. They should run the length of the creator's life, period. I can't think of a single reason somebody's children should benefit from their parents' creativity, aside from growing up under its benefit.